Nitrogen sorption

ABSTRACT

Nitrogen-sorbing and -desorbing compositions and methods of using the same are disclosed, which are useful for the selective separation of nitrogen from other gases, especially natural gas.

The government has rights in this invention pursuant to Contract No.DE-FG03-90ER80892 awarded by the Department of Energy.

BACKGROUND OF THE INVENTION

It has been estimated that 25% of the natural gas in the United Statescontains unacceptably high levels of the non-combustive contaminantnitrogen. Efforts to remove nitrogen from natural gas have includedmethane sorption and various techniques of cryogenic distillation suchas liquification, turbocryogenic distillation, and "cold box"separation. All such efforts, though successful, have been relativelyexpensive and inefficient. There thus exists a need for a simple,efficient and low cost method of selectively removing nitrogen fromnatural gas. This need and others are met by the present invention,which is summarized and described in detail below.

SUMMARY OF THE INVENTION

The present invention comprises a nitrogen-absorbing and -desorbingcomposition (also referred to herein as a "sorption material") and aprocess of using the same to selectively remove nitrogen from othergases.

More particularly, the sorption composition comprises an organometalliccomplex either alone or in a relatively inert matrix wherein the matrixis either a liquid capable of dissolving the organometallic complex to≧0.1M, or a solid such as a polymer or a porous inorganic material, theorganometallic complex comprising a transition metal and at least oneligand capable of providing five or six coordinating atoms. In somecases, one ligand is in the axial position and is termed an "axialbase," the axial base being capable of providing a coordinating atom tothe organometallic complex.

The process comprises absorbing nitrogen from a nitrogen-containing feedstream typically containing substantially no oxygen, no carbon monoxide,no thiols and no sulfides by contacting the feed stream with thenitrogen-sorption and -desorption material, followed by desorbingnitrogen from the sorption material. Desorption may be accomplished bytemperature swing, pressure swing or a combination of the two. As thenitrogen-sorption capacity decreases over time due to decomposition ofthe sorption material, an optional step to improve efficiency isregeneration of its nitrogen-sorption capacity by various methods.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic of an exemplary pressure swingabsorption/desorption process of the present invention.

FIG. 2 is a schematic of an exemplary hybrid pressure/temperature swingabsorption/desorption process of the present invention.

FIG. 3 is a schematic of the exemplary process depicted in FIG. 1wherein a pressure-reducing turbine and a regeneration loop areincluded.

FIGS. 4 and 5 are graphs of isotherms observed for two exemplarysorption compositions of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, there is provided anitrogen-absorbing and -desorbing material having utility in theselective removal of nitrogen from a broad class of other gases andspecific utility in the removal of nitrogen from naturally-occurringnatural gas mixtures.

According to a preferred embodiment, the sorption material is a solutionhaving two essential components: a solvent and an organometallic complexthat is soluble in the solvent to ≧0.1 M. In general terms, the solventshould have the following properties:

hydrophilic, with a solubility parameter of >20 MP-a^(1/2) andpreferably ≧30 MPa^(1/2) ;

either incapable of coordinating or capable of only weakly coordinatingwith the nitrogen-binding site of the organometallic complex;

solubility of the organometallic complex therein should be ≧0.1M,preferably ≧0.25M, but not to exceed 95% of the solubility limit at theminimum operating temperature or that concentration that gives asolution viscosity ≦100 cps at the operating temperature; and

leads to a nitrogen-absorbing solution having an apparent nitrogensolubility of ≧0.1 mol nitrogen per mol organometallic complex under thetemperature and pressure of the nitrogen-containing feed as it enters asorption column, and a substantially diminished nitrogen solubilityunder the temperature and pressure conditions prevailing in a desorptionor stripping column.

Preferably, solvents should also have low volatility (b.p. 22 90° C.)and low toxicity.

Especially preferred solvents are water, dimethyl formamide (DMF),dimethyl acetamide (DMAc), formamide, N-methylformamide, glycerol, andglycols, such as ethylene glycol, propylene glycol, butylene glycol,dimethylethylene glycol and glycolic oligomers. Generally speaking,useful solvents include liquids or mixtures of the same which arepreferably polar and hydrophilic, although non-polar liquids may beuseful in some cases. Classes of useful solvents include lactams,sulfoxides, nitriles, amides, amines, esters, and ethers. In addition tothe preferred solvents mentioned above, preferred examples of the broadclasses of solvents include dimethylsulfoxide (DMSO), diethylsulfoxide,propylene carbonate, ethylene carbonate, benzonitrile, tributylphosphate(TBP) and other phosphates, alcohols, glycols, N-ethylformamide andnitrogen-containing heterocycles.

The complex comprises at least one, but not more than six, ligand(s)with a transition metal. The ligand(s) must be capable of providing fiveor six coordinating atoms to the transition metal. The ligand(s) may bemonodentate, bidentate, tridentate, tetradentate, pentadentate orhexadentate, or any combination of mono-, bi-, tri-, tetra-, penta- orhexadentate that forms a pentacoordinate or a hexacoordinate complexwith the metal. The organometallic complex is preferablypentacoordinate, with bound nitrogen occupying the sixth coordinationsite. When the bound nitrogen displaces one of the ligands, theorganometallic complex may be hexacoordinate.

Preferred transition metals that comprise part of the organometalliccomplex include the metals of Groups 7, 8 and 9, such as the early andthird row transition metals Mo(O), W(O), Re(I), Re(II), Ru(II), Fe(I),Fe(II), Co(O), Co(I), Os(II), Ir(I), Rh(I) and Mn(I). Other, lesspreferred transition metals include the metals of Groups 3, 4, 5 and 6.In general, those metals in Groups 3-5 with high oxophilicity andconsequent susceptibility to irreversible oxidation should be avoided.Also, the dinitrogen complexes of the metals in Groups 3 through 6 aregenerally less preferred as they tend to be susceptible to chemicalreaction (such as protonation) at the coordinated dinitrogen, which maylead to loss of nitrogen-binding capability of the organometalliccomplex.

The ligand that is trans to the coordinated nitrogen is termed the"axial base." Although the axial base is usually a different moiety thanthe equatorial ligands, it may in fact be the same. Exemplary axialbases are selected from halogens and pseudohalogens (such as hydride,cyanide and thiocyanate ions), arsines, stibnines, phosphines,phosphites, thiols, sulfides, nitrogen-containing bases, includingheterocycles such as pyridines, imidazoles, amides and amines,sulfur-containing heterocycles such as thiophenes, carbon monoxide,nitrogen, nitrous oxide, hydroxy, alkoxy, aryloxy, hydrocarbon residuessuch as alkyl, aryl and olefinic groups. The axial base may also becovalently attached to one or more of the equatorial ligands through abridging group. A tabulation of suitable axial bases is set forth inTable 1. Table 2 contains definitions of the R substituents of both theaxial bases and the polydentate ligands, while Table 3 containsdefinitions of the R' bridging groups of the polydentate ligands.

                  TABLE 1                                                         ______________________________________                                        Group No.                                                                              Structure   Classes of Compounds                                     ______________________________________                                                  ##STR1##   amines, phosphines, arsines and stibnines where Z is                          N, P, As, Sb and R is H or as defined in Table 2,                             Substituent Group A, B or C                              2        R-S-R       thiols and sulfides where R is                                                H or as defined in Table 2,                                                   Substituent Group A, B or C,                                                  excluding H.sub.2 S and provided that                                         when R is alkyl, it contains                                                  ≧4 carbons                                        3                                                                                       ##STR2##   N-contg. aromatic and nonaromatic heterocycles,                               including substituted and unsubstituted pyrroles,                             pyra- zines, pyrimidines, pyridines and imidazoles                            where R is H or as defined in Table 2, Substituent                            Group A, B or C                                          4                                                                                       ##STR3##   S-contg. aromatic heterocycles, including                                     substituted and unsubstituted thiophenes, tetrahydrot                         hiophenes and thiazoles where R is H or as defined                            in Table 2, Substituent Group A, B or C                  5        OR          hydroxy, alkoxy and aryloxy                                                   where R is H or as defined                                                    in Table 2, Substituent                                                       Groups A, B or C                                         6        X           halogens and pseudohalogens                                                   where X is F.sup.˜, Cl.sup.˜,                                     Br.sup.˜, I.sup.˜,                                                H.sup.˜, CN.sup.˜  and SCN.sup.˜       7        CO, NO      carbon monoxide and nitrous                                                   oxide                                                    ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Substituent                                                                   Group    Type        Definition of R                                          ______________________________________                                        A        alkyl and   1°, 2°, 3° and cyclic contg.                 substituted 1-10 carbons where substitu-                                      alkyl       ents are selected from halo,                                                  hydroxy, cyano, amido, amino,                                                 mono- and dialkylamino,                                                       mono- and diaryl amino,                                                       mercapto, sulfonyloxy,                                                        alkoxy, thioalkoxy, aryloxy,                                                  thioaryloxy, carboxy,                                                         alkoxycarbonyl, alkyl- and                                                    arylsulfinyl, alkyl- and                                                      arylphospho, alkyl- and                                                       arylphosphono, substituted                                                    and unsubstituted aryls,                                                      including phenyl, biphenyl,                                                   naphthyl, substituted and                                                     unsubstituted N- and S-contg.                                                 heteroaryl, including                                                         pyridyl, pyrryl, piperidinyl,                                                 piperazyl, thienyl, tetra-                                                    hydrothioenyl, and thiazolyl                                                  groups                                                   B        aryl and    phenyl, biphenyl, naphthyl                                        substituted and anthracenyl where sub-                                        aryl        stituents are selected from                                                   those in this Table,                                                          Substituent Group A                                      C        heterocycles                                                                              N- and S-contg. hetero-                                           and substi- cycles as defined in                                              tuted       Table 1, Groups 3 and 4,                                          heterocycles                                                                              where substituents are                                                        selected from those in this                                                   Table, Substituent Group A                               ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        Bridging                                                                      Group     Type        Definition of R'                                        ______________________________________                                        I         alkylene,   1°, 2°, 3° and cyclic, contg.                substituted 1-10 carbons where bridging                                       alkylene,   hydrocarbon chain contains                                        alkenylene  1-4 carbons and where sub-                                        and substi- stituents are selected from                                       tuted       those in Table 2,                                                 alkenylene  Substituent Group A                                     II        arylene and as defined in Table 2,                                            substituted Substituent Group B, and                                          arylene     contg. two coordinating/                                                      chelating groups selected                                                     from N, P, S, As and Sb in                                                    the 1,2-positions for phenyl;                                                 in the 1,2-, 1,8- or 2,3-                                                     positions for naphthyl; in                                                    the 2,2'-, 2,3- or 3,4-                                                       positions for biphenyl; or                                                    in the 1,2-, 2,3- or 1,9-                                                     positions for anthracenyl                               III       heterocycles                                                                              as defined in Table 2,                                            and substi- Substituent Group C, and                                          tuted       contg. two coordinating/                                          heterocycles                                                                              chelating groups selected                                                     from N, P, S, As and Sb in                                                    any two adjacent positions                              ______________________________________                                    

Suitable monodentate equatorial ligands include the following fourgroupings of organic compounds:

1. arsines, amines, phosphines and stibnines of the structure ##STR4##where Z is selected from As, N, P and Sb and each R is independentlyselected from --H or any of the substituents recited in Table 2,Substituent Group A, B or C (as a group, the three R substituents maycomprise any combination of --H or the substituents shown in Table 2);

2. thiols an sulfides of the structure

    R--S--R

where R is as defined above

3. halogens and the pseudohalogens H⁻, CN⁻ and SCN⁻ ; and

4. carbon monoxide and nitrous oxide.

Suitable bidentate equatorial ligands include the following four groupsof organic compounds:

1. amines, arsines, phosphines and stibnines of the structure ##STR5##where substituted and Z are as defined above and R' is any of thebridging ligands set forth in Table 3;

2. phosphites of the structure ##STR6## where R and R' are as definedabove;

3. thiols and sulfides of the structure

    R--S--R'--S--R

where R and R' are as defined above; and 4. substituted andunsubstituted nitrogen- and sulfur-containing heterocycles as defined inTable 1, Groups 3 and 4.

Suitable tridentate equatorial ligands include the following four groupsof organic compounds:

1. amines, arsines, phosphines and stibnines of the structure ##STR7##where R, R' and Z are as defined above;

2. phosphites of the structure ##STR8## where R and R' are as definedabove;

3. thiols and sulfides of the structure

    R--S--R'--S--R'--S--R

where R and R' are as defined above; and

4. substituted and unsubstituted nitrogen- and sulfur-containingheterocycles as defined in Table 1, Groups 3 and 4.

Suitable tetradentate equatorial ligands include the following sixgroups of organic compounds:

1. amines, arsines, phosphines, and stibnines of the structure ##STR9##where Z, R and R' are as defined above;

2. phosphites of the structure ##STR10## where R and R' are as definedabove;

3. thiols and sulfides of the structure ##STR11## where R and R' are asdefined above.

4. substituted and unsubstituted nitrogen- and sulfur-containingheterocycles as defined in Table 1, Groups 3 and 4.

5. substituted and unsubstituted porphyrins of the structure ##STR12##where R is as defined above; and

6. substituted and unsubstituted phthalocyanines of the structure##STR13## where R is as defined above.

Structural drawings of various equatorial ligands ("L"), where theligand trans to the bound nitrogen (N₂) is an axial base ("B")coordinated to the metal ("M") are shown below: ##STR14##

The ligands (including the axial base) may be in any combination suchthat they provide 5 or 6 coordinating atoms to the complex. Thus, forexample, any of the following combinations of ligands are suitable: 5 or6 monodentates; 3 or 4 monodentates and 1 bidentate; 1 each of a mono-,bi- and tridentate; 1 each of a bi- and tetradentate; 1 each of a mono-and a pentadentate, 2 or 3 monodentates and 1 or 2 tridentate(s); 1 or 2monodentate and 1 tetradentate; 3 bidentates; 1 pentadentate; and 1hexadentate.

As mentioned above, the nitrogen-sorbing and -desorbing material may bea solid or a solution. When in the form of a solid, the nitrogensorption material may be either a solid organometallic complex of thestructures discussed above or such an organometallic complex in arelatively inert solid matrix. By an "inert" matrix is meant a materialthat is substantially non-reactive and does not absorb substantialamounts of nitrogen or other gases in the feed. Preferably, the matrixis highly permeable to gaseous nitrogen so as to permit rapid diffusiontherethrough. One class of suitable matrices comprises polymers;examples of such include polydimethylsiloxane,poly(trimethylsilylpropyne), polyurethane, polyvinylalcohol,polyvinylacetate, and cellulose esters. Another class of suitablematrices comprises a porous inorganic material such as an oxide or aceramic; examples of suitable inorganic oxides include silicon dioxideand titanium dioxide.

The nitrogen sorption material may be used in any of a pressure swingabsorption (PSA), a temperature swing absorption (TSA) or a hybridcombination of PSA and TSA. In general, when the nitrogen sorptionmaterial is in the form of a solution, it is preferably used in a PSAmode. In a PSA mode, the difference in nitrogen partial pressuresbetween the absorption and desorption steps is preferably in the rangeof 10 to 400 psi. Nitrogen partial pressure in the desorption step mayalso be reduced by the use of an inert sweep gas, such as carbondioxide, argon, hydrogen, helium or methane, preferably in acountercurrent flow mode. Sweep gas may also effectively be generated insitu by the release of other gases (such as methane or otherhydrocarbons) absorbed in the solution or by solvent vapor; this releaseof other sorbed gases effectively lowers the partial pressure ofnitrogen. In terms of total pressure, the absorption step is preferablyconducted at a total pressure that is at least 20 times the totalpressure of the desorption step. When used in a TSA mode, the preferredtemperature differential between the absorption and desorption steps isin the range of 20° to 100° C. for economic efficiency to be realized.

The feed gas preferably comprises a mixture of nitrogen and other gases,typically methane and other hydrocarbons, the mixture preferablycontaining essentially no oxygen, no carbon monoxide, no thiols and nosulfides. Preferred limits on such impurities are such that the partialpressures of the gases are as follows: oxygen ≦1 psi, preferably 10⁻³psi; carbon monoxide ≦10 psi; sulfides and thiols ≦10hu -3 psi.Notwithstanding these preferred lmits, in some cases the nitrogensorption material may be relatively unaffected by the presence of suchimpurities and so the feed gas may contain substantial amounts, say, upto 10 vol %, of the same. In general, non-nitrogen components should besoluble in the solvent to a concentration that is less than twice thesolubility of the organometallic complex. The feed may be at virtuallyany temperature in the range of -20° C. to 100° C. although in certaincases, mentioned below, higher temperatures may also be used. Ingeneral, the preferred temperature range is 0° C. to 100° C. The amountof nitrogen in the feed stream may be anywhere from 0.1 to 80 vol %.Nitrogen may be mixed with virtually any other gas or combination ofgases with the restrictions on impurities noted above. Preferredapplications include mixtures of nitrogen with hydrocarbons containingfrom 1 to 7 carbons, including natural gas, and with hydrocarbons frompartial oxidation of hydrocarbons containing from 1 to 7 carbon atoms(from the oxidation of coal and from the oxidative coupling ofhydrocarbons). The range of the temperature of the feed may be from 0°C. to 200° C., preferably 20° C. to 150° C. The feed may be fed at apressure of anywhere from 20 psi to 2000 psi.

Over time, the nitrogen-sorbing capacity of the solution may decreasedue to a formal oxidation of the metal atom in the organometalliccomplex. The nitrogen-absorbing capability of the solution may beperiodically regenerated by a variety of techniques, including:

(1) formally reducing the metal by heating the solution to 30° C. to180° C. while avoiding oxidizing conditions, preferably in the presenceof a reducing agent such as hydrogen, magnesium, iron or thiosulfateion;

(2) stripping the solvent from the solution and then recrystallizing theresidual organometallic complex from a suitable solvent under a nitrogenor other inert gas atmosphere; and

(3) demetallating the organometallic complex in solution by the additionof a strong acid, extracting the oxidized transition metal into animmiscible organic solvent, then coordination of the reduced transitionmetal with the solution of the equatorial ligand(s) and axial base, andrecrystallizing the regenerated organometallic complex.

In connection with the first regeneration method mentioned above,oxidizing conditions may be avoided by heating the solution (a) under avacuum of from 0.2 to 20 cmHg for about 1 to 48 hours, (b) in an inertatmosphere such as nitrogen or argon for about 1 to 72 hours, or (c) ina reducing atmosphere such as hydrogen for from about 1 to 72 hours,with or without the presence of a reduction catalyst such as a platinumgroup metal.

In connection with the second regeneration method, the inactiveorganometallic complex may be isolated from the solvent by vacuum oratmospheric distillation of the solvent, and the residual organometalliccomplexes recrystallized from an appropriate solvent.

In connection with the third method of regeneration, suitable strongacids include hydrochloric acid, sulfuric acid, and trifluoroaceticacid. The oxidized metal may be extracted into an immiscible organicsolvent, such as toluene and other aromatic solvents, and hexane andother aliphatic solvents, by addition of an organic-soluble metalextractant, such as dialkylphosphoric acids, alkylamines, quaternaryalkylamines, and alkyl-β-diketones, to the aromatic or aliphaticsolvent. Suitable solvents for recrystallization of the organometalliccomplex include water, methanol, ethanol, tetrahydrofuran, andacetonitrile.

Referring now to the drawings, wherein like numerals refer to the sameelements, use of the solution of the present invention in a PSA mode isdepicted in FIG. 1. There, a nitrogen-containing feed 10 is introducedinto a conventional gas-liquid absorption column 20 so that the gas isefficiently contacted with the solution of the present invention. Withinthe absorption column 20, nitrogen is selectively absorbed by thesolution, resulting in a reduction in the nitrogen concentration in the"product" gas 25 exiting the column (it being understood that virtuallyany gas other than nitrogen, depending upon the desired separation,could be regarded as the product gas). The residence time of the solventin the absorption column 20 is on the order of a few minutes andgenerally should be sufficiently long to achieve nitrogen binding to atleast 10 mol % of the organometallic absorbent. The column should besized sufficiently to accommodate the requisite volume and flow rate ofliquid absorbent to have sufficient contact time for nitrogen to beabsorbed by the liquid. In place of the absorption column 20, othergas-liquid contactors may be utilized, such as membrane contactors inthe form of hollow fiber modules. The nitrogen-complexed liquidabsorbent 28 is passed to a stripping column 40 in which nitrogen isdesorbed from the liquid absorbent. For nitrogen desorption to occur inthe stripping column, the partial pressure of nitrogen in thenitrogen-containing stream 45 exiting the stripping column 40 must beless than the partial pressure of nitrogen in the product stream 25exiting the absorption column 20. This condition is met by operating thestripping column 40 at a reduced pressure relative to the absorptioncolumn 20 (typically near 0 psig total pressure) or by using a sweepstream 35 to maintain low nitrogen partial pressures in thenitrogen-containing stream 45 exiting the stripping column 40. Thenitrogen-containing stream 45 desorbed from the liquid absorbent exitsthe stripping column 40 at substantially the same pressure as thatprevailing in the stripping column, which is typically near 0 psig totalpressure. In some cases the desorbed nitrogen from thenitrogen-containing stream 45 may be the end product of the separationprocess. After nitrogen is desorbed from the liquid absorbent in thestripping column 40, the nitrogen-stripped liquid absorbent 48 isreturned to the absorption column 20 by use of a pump 30, and the cycleis repeated.

Use of the nitrogen-sorbing and -desorbing solution of the presentinvention in a hybrid PSA/TSA mode is shown schematically in FIG. 2.There, the system is operated in generally the same manner as describedfor FIG. 1, except that the stripping column 40 is operated at anelevated temperature relative to the absorption column 20, the additionof heat to the stripping column 40 being depicted schematically by thesymbol "+Q". Alternatively, the absorption column 20 may be cooledrelative to the stripping column 40, this being schematically depictedby the symbol "-Q". This hybrid mode of operation is useful incompensating for the fact that the nitrogen-binding capacity of theliquid absorbent for a given nitrogen partial pressure decreases withincreasing temperature inasmuch as the nitrogen-binding is typically asomewhat exothermic reaction. As a result, the nitrogen partial pressurein equilibrium with the nitrogen-containing absorbent will increase withincreasing temperature. For nitrogen desorption to occur in thestripping column 40, the concentration in the absorbent liquid inequilibrium with product gas 25 exiting the absorption column 20 at thetemperature and pressure prevailing therein must exceed theconcentration of nitrogen in the absorbent in equilibrium with thenitrogen in nitrogen-containing stream 45 at the temperature andpressure prevailing in the stripping column 40. The advantage of thehybrid PSA/TSA mode over the purely PSA mode is that in the former,nitrogen desorption can be achieved in the stripping column 20 atnitrogen partial pressures greater than those allowed in the strictlyPSA mode. As with the PSA mode, the hybrid PSA/TSA mode may be used toachieve nitrogen desorption in the stripping column 40 by eitheroperating the stripping column at reduced pressure relative to theabsorption column or by the use of a sweep gas. However, since thestripping column is at a higher temperature than the absorption column,the stripping column need not be at a lower pressure but may be at thesame or even higher pressure than the absorption column. Anotheradvantage of operating the stripping column at elevated temperature isthat an increase in the rate of nitrogen desorption from the liquidabsorbent occurs, resulting in a decrease in the residence time requiredfor the liquid absorbent in the stripping column.

FIG. 3 depicts the inclusion of a regeneration loop 50 wherein thenitrogen-stripped liquid sorbent 48 is treated by one of the methodsdescribed above to regenerate its nitrogen-sorption capacity, as well asthe inclusion of a pressure-reducing turbine 38 to recover energyotherwise lost, the energy being used to drive the liquid pump 30. Apreferred type of pressure-reducing or power recovery turbine is thatwhich is commercially available from Sulzer Bingham of Portland, Oreg.

When the nitrogen-absorbing and -desorbing material is a solid, thematerial can be used in essentially the same manner as that describedabove for liquid absorbents except that the gas-liquid contactors wouldconstitute fluidized beds with the solid material and either feed gas ordesorbed gas from the stripper preferably flowing countercurrently. Thesolid may also be used in conventional pressure-swing ortemperature-swing processes in a manner similar to the way zeolites orcarbon molecular sieves are used to separate gas mixtures such as in theproduction of nitrogen or oxygen from air.

EXAMPLE 1

A 0.25M solution of the organometallic complex [Ru(H₂ O)(Hedta)]⁻ inwater (as the potassium salt) was used to determine nitrogen-bindingisotherms at 21° C. and at 41° C. (Hedta is monoprotonated ethylenediamine tetraacetate). This was accomplished by measuring the uptake ofnitrogen by the solution over a range of nitrogen pressures. Theresulting isotherms are shown in FIG. 4. From these isotherms, theequilibrium constant of nitrogen binding is calculated to be 0.08(psi-M)⁻¹ at 21° C. and 0.024 (psi-M)⁻¹ at 41° C. These equilibriumconstants are based on the following assumed nitrogen-binding reaction:

    2[Ru(H.sub.2 O)(Hedta)].sup.- +N.sub.2 ⃡([Ru(N.sub.2)(Hedta)].sub.2 N.sub.2).sup.-2 +2H.sub.2 O.

These data show that at 21° C. and high nitrogen pressures (300 to 350psi), the solution of [Ru(H₂ O)(Hedta)]⁻ absorbs approximately 0.5 mmoleof nitrogen per mmole of the complex. However, at 21° C. and lownitrogen pressure (<50 psi), the solution absorbs <0.2 mmole nitrogenper mmole of the complex. Thus, an aqueous solution of thisorganometallic complex can be used to remove nitrogen from ahigh-pressure gas stream by first permitting the solution to absorbnitrogen at high pressure, then pumping the nitrogen-laden solution to astripping column at low pressure to allow nitrogen to desorb from thesolution. For example, at a solution temperature of about 21° C., aswing in nitrogen partial pressure from 350 psi in the absorption stageto 50 psi in the desorption stage will result in the net removal of morethan 0.3 mmole of nitrogen per mmole of [Ru (Hedta)]⁻.

Referring again to FIG. 4, the 41° C. isotherm is shifted below the 21°C. isotherm because coordination of nitrogen to the organometalliccomplex is an exothermic process, i.e., as the temperature increases,the fraction of nitrogen coordinated to the organometallic complexdecreases. This property may be used to increase the amount of nitrogenthat is desorbed from the nitrogen-laden solution. Accordingly, a hybridPSA-TSA process may be used to remove nitrogen from a gas stream bypermitting the complex-containing solution to absorb nitrogen at highpressure at about 21° C., removing the nitrogen-laden solution, andheating it to about 41° C., then pumping the nitrogen-laden solution toa stripping column at low pressure, thereby allowing nitrogen to desorbfrom the solution. For example, by maintaining the nitrogen partialpressure at about 350 psi and the temperature at 21° C. in theabsorption column and at about 50 psi and 40° C. in the strippingcolumn, about 0.45 mmole nitrogen per mmol [Ru (Hedta)]⁻ may be removedfrom the feed gas.

EXAMPLE 2

The nitrogen-binding isotherm at 21° C. of a 0.02M solution of theorganometallic complex [Fe(H)(diphos)₂ ]⁺ (diphos=Ph₂ PCH₂ CH₂ PPh₂) wasdetermined. This was accomplished by measuring, as a function ofnitrogen pressure, the intensity of the infrared absorption bandcorresponding to the nitrogen-nitrogen stretch at 2122 cm⁻¹. The resultsare shown in FIG. 5. From this isotherm the equilibrium constant fornitrogen-binding is calculated to be 0.15 (psi-M)⁻¹. The value of theequilibrium constant is based on the following assumed nitrogen-bindingreaction:

    [Fe(H)(diphos).sub.2 ].sup.+ +N.sub.2 ⃡[Fe(H)(diphos).sub.2 (N.sub.2)].sup.+.

Since [Fe(H)(diphos)₂ ] binds nitrogen more tightly than [Ru(H₂O)(Hedta)]⁻, the nitrogen can be desorbed only at relatively lownitrogen partial pressure and/or elevated temperatures. For example, toachieve a net removal of 0.5 mmole nitrogen per mmole [Fe(H)(diphos)₂ ]⁺at a temperature of about 21° C., the nitrogen partial pressure in thedesorption column would have to be about 7 psia or less given a nitrogenpressure in the absorption column of at least 200 psig. As is the casewith [Ru(H₂ O)(Hedta)]⁻, desorption of nitrogen from a solution of[Fe(H)(diphos)₂ ]⁺ can be achieved at higher nitrogen pressures if thesolution is heated (e.g., to about 41° C.) during the desorption ofnitrogen.

EXAMPLE 3

To demonstrate that nitrogen is selectively absorbed from a gas mixturecontaining nitrogen and methane, a 0.36M (7.2 mmole) aqueous solution ofthe organometallic complex [Ru(H₂ O)(Hedta)]⁻ (as the potassium salt)was exposed to a 7.5% nitrogen-containing nitrogen/methane gas mixtureat 518 psi. Nitrogen partial pressure loss demonstrated thatapproximately 0.24 mmole of nitrogen gas was complexed or absorbed bythe complex-containing solution. Isolation of the remainder of the gasmixture and analysis by gas liquid chromatography demonstrated itscomposition to be 7.0±0.1% nitrogen, indicating that 0.5% (0.19 mmole)nitrogen was desorbed from the solution.

EXAMPLE 4

The organometallic complex-containing solution of the present inventionwas demonstrated to be regenerable. A 0.4M aqueous solution of theorganometallic complex [Ru(H₂ O)(Hedta)]⁻ was exposed to air for 24hours. Aerobic oxidation of [Ru(H₂ O)(Hedta)]⁻ is reported in theliterature to yield [Ru(H₂ O)(Hedta)], which does not absorb nitrogen.That is, contact with air results in oxidation of Ru from the +2 to the+3 oxidation state. Subsequently the oxidized solution was contactedwith the reducing agent magnesium to regenerate [Ru(H₂ O)(Hedta)]⁻. Theregenerated solution was found to reversibly absorb nitrogen accordingto the binding isotherm shown in FIG. 4 and also to selectively absorbnitrogen from N₂ /CH₄ as described in Example 3.

EXAMPLE 5

The capability of the organometallic complex of the present invention ina solid form to reversibly bind nitrogen was demonstrated. A 1.86 g(1.99 mmole) portion of the solid organometallic complexMo(triphos)[P(CH₃)₂ (C₆ H₅)]₂ (triphos=(C₆ H₅)₂ PCH₂ CH₂ P(C₆ H₅)CH₂ CH₂P(C₆ H₅)₂) was exposed to a stream of pure nitrogen and indicatednitrogen absorption by turning a bright orange color. The solid complexwas then heated under vacuum (1 torr) for 3 days at 50° C., causing thebright-orange solid to change to deep orange-brown. This color change isindicative of the complex desorbing nitrogen and forming the putativefive-coordinate complex Mo(triphos)[P(CH₃)₂ (C₆ H₅)]. A portion(1.59±0.01 g or 1.76±0.01 mmole) of this orange-brown solidorganometallic complex was then placed in Fischer-Porter pressure bottleunder 32 psig nitrogen at 20°±2° C. Over a 25 hour reaction period,1.22±0.14 mmole nitrogen was absorbed by the solid complex(corresponding to a 4.5±0.5 psig nitrogen partial pressure loss) and theinitial bright orange color of the complex was regenerated. The solidcomplex was then weighed at 1.62±0.01 g, thus exhibiting an overallweight increase of 0.03±0.02 g, which corresponds to an absorption of1.1±0.7 mmole nitrogen.

The composition of the present invention may also be used strictly inthe absorption mode, e.g., as a nitrogen detector or a nitrogen getterto remove small amounts of nitrogen from inert gas streams such as anargon gas stream.

The terms and expressions which have been employed in the foregoingspecification are used therein as terms of description and not oflimitation, and there is no intention in the use of such terms andexpressions of excluding equivalents of the features shown and describedor portions thereof, it being recognized that the scope of the inventionis defined and limited only by the claims which follow.

What is claimed is:
 1. A process for the separation of nitrogencomprising the steps:(a) absorbing nitrogen from a nitrogen-containingfeed stream containing from 0.1 to 80 vol % nitrogen by contacting saidfeed stream with a composition comprising a solvent and anorganometallic complex, said organometallic complex consistingessentially of a transition metal selected from Mo, W and a metal fromGroups 7, 8 and 9, and one or more ligands capable of providing five orsix coordinating atoms to said organometallic complex, said solventbeing capable of dissolving said organometallic complex to ≧0.1M; and(b) desorbing nitrogen from said composition to a product stream whereinthe conditions for conducting steps (a) and (b) are selected from oneof(i) step (a) is conducted at a lower temperature than step (b); (ii)step (a) is conducted at a nitrogen partial pressure that is greaterthan the nitrogen partial pressure of step (b); and (iii) both (i) and(ii).
 2. A process for the separation of nitrogen comprising thesteps:(a) absorbing nitrogen from a nitrogen-containing feed streamcontaining from 0.1 to 80 vol % nitrogen by contacting said feed streamwith a solid composition comprising a solid inert matrix and anorganometallic complex, the concentration of said organometallic complexbeing ≧0.1M relative to said matrix, said organometallic complexconsisting essentially of a transition metal selected from Mo, W and ametal from Groups 7, 8 and 9, and one or more ligands capable ofproviding five or six coordinating atoms to said organometallic complex;and (b) desorbing nitrogen from said composition to a product stream,wherein the conditions for conducting steps (a) and (b) are selectedfrom one of(i) step (a) is conducted at a lower temperature than step(b); (ii) step (a) is conducted at a nitrogen partial pressure that isgreater than the nitrogen partial pressure of step (b); and (iii) both(i) and (ii).
 3. The process of claim 1 or 2 wherein said one or moreligands comprises a ligand selected from a monodentate, a bidentate, atridentate, a tetradentate, a pentadentate, and a hexadentate ligand. 4.The process of claim 1 or 2 wherein said composition includes an axialbase as one of the ligands.
 5. The process of claim 1 or 2 wherein thedifference between the temperatures of steps (a) and (b) is 20° to 100°C.
 6. The process of claim 1 or 2 wherein the difference between thenitrogen partial pressures of steps (a) and (b) is 10 to 400 psi.
 7. Theprocess of claim 1 or 2 wherein step (a) is conducted at a totalpressure that is at least 20 times the total pressure in step (b). 8.The process of claim 1 or 2 wherein step (a) is conducted at a lowertemperature than the temperature of step (b).
 9. The process of claim 8wherein the difference between the temperatures of steps (a) and (b) is20° to 100° C.
 10. The process of claim 1 or 2 wherein step (b) isconducted with a sweep gas.
 11. The process of claim 10 wherein saidsweep gas is selected from argon, carbon dioxide, helium, hydrogen, andmethane.
 12. The process of claim 1 or 2 wherein said feed streamcontains oxygen in an amount such that the oxygen partial pressure insaid feed stream is ≦1 psi.
 13. The process of claim 1 or 2 wherein saidfeed stream comprises nitrogen and hydrocarbon gas predominantlycontaining 1 to 7 carbon atoms.
 14. The process of claim 13 wherein saidhydrocarbon gas is natural gas.
 15. The process of claim 13 wherein saidhydrocarbon gas is from a partial oxidation of hydrocarbons.
 16. Theprocess of claim 13 wherein said hydrocarbon gas is from partialoxidation of coal.
 17. The process of claim 13 wherein said hydrocarbongas is from oxidative coupling of hydrocarbons.
 18. The process of claim1, including periodic regeneration of the nitrogen-absorbing capabilityof said solution.
 19. The process of claim 18 wherein said regenerationis accomplished by heating said solution in an inert atmosphere.
 20. Theprocess of claim 18 wherein said regeneration is accomplished by heatingsaid solution in the presence of a reducing agent.
 21. The process ofclaim 20 wherein said reducing agent is selected from hydrogen, iron,magnesium and a platinum group metal.